In computer science, a virtual machine (VM) is a portion of software that, when executed on appropriate hardware, creates an environment allowing the virtualization of an actual physical computer system. Each VM may function as a self-contained platform, running its own operating system (OS) and software applications (processes). Typically, a virtual machine monitor (VMM) manages allocation and virtualization of computer resources and performs context switching, as may be necessary, to cycle between various VMs.
A host machine (e.g., computer or server) is typically enabled to simultaneously run multiple VMs, where each VM may be used by a local or remote client. The host machine allocates a certain amount of the host's resources to each of the VMs. Each VM is then able to use the allocated resources to execute applications, including operating systems known as guest operating systems. The VMM virtualizes the underlying hardware of the host machine or emulates hardware devices, making the use of the VM transparent to the guest operating system or the remote client that uses the VM.
Often times, a VM may need to be migrated from one host machine to another host machine for a variety of reasons. This migration process may be a “live” migration process, referring to the fact that the VM stays running and operational (i.e., “live”) during most of the migration process. During live migration, the entire state of a VM is transferred from one host machine to another host machine. A critical piece of this transmission of state is the transfer of memory of the VM. The entire memory of a VM can often times fall in the order of gigabytes, which can result in a lengthy live migration transfer process. In addition, because the VM is “live” during this transfer, memory may become “dirty” during the transfer. This means that a particular page of the memory that was already transferred has been modified on the VM that is still residing on the source host. Typically, these “dirty” pages are marked so that those particular pages of memory can be transferred again during the live migration process.
Currently, a problem with the live migration occurs when the rate of state change for a VM is faster than the rate of migration. For instance, with respect to memory, live migration will not be completed as long as the rate of pages being dirtied is faster than the rate of page migration. The solutions to this problem have either involved (1) continue transfer pages and dirtying pages at the full speed of the VM and hope that migration will eventually complete, or (2) stop the VM and complete migration. These solutions are implemented without regard to the number of CPUs on a migrating VM or an ability to limit computing resources implemented by the VM.